Fast handover method for cross sector scenario in mobile communication systems

ABSTRACT

A method for handover in a mobile communication system, wherein cell search is performed by determining cell identities based on cell identity information transmitted within downlink data is provided. The cell power of cells detected by cell search is measured. Moreover, the cell power of an additional cell is measured, the additional cell having a cell identity that is related to a cell identity of a cell detected by cell search in that the additional cell and the cell detected by cell search are adjacent cells. The method is applicable in cross-sector scenarios avoiding call drops due to lengthy cell detection.

This application is a continuation of U.S. patent application Ser. No.13/832,947, filed Mar. 15, 2013, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

In a mobile communication system, a mobile device wishing to access acell first undertakes a cell search procedure. In a mobile communicationsystem, such as LTE, the cell search procedure comprises a series ofsynchronization stages by which the mobile device (UE in the terminologyof LTE) determines time frequency parameters that are necessary todemodulate the downlink and to transmit uplink symbols with the correcttiming. Specifically, each cell in the system needs to be uniquelyidentified if the mobile device wishes to connect to a cell or if themobile device is already connected to a cell but wishes to connect toanother LTE cell. The latter scenario is referred to as a handover. Forthis purpose the base station transmits its cell-id within downlinkdata. The cell-id of the base station is detected by the mobile devicewhen performing cell search. However, the handover procedure hasproblems in a cross sector scenario when the mobile device is moving ina fast speed as the cell-id detection takes some time resulting in acall drop. The cell-id of an adjacent cell to which the mobile devicewishes to connect cannot be easily detected in advance to crossing thesector boundary as the beam of the adjacent cell may not be visible tothe mobile device due to sectored beam forming of the mobile station.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made to the accompanying drawings.

FIG. 1 shows a diagram of an LTE system.

FIG. 2 shows a PSS and SSS frame and slot structure in the FDD case ofan LTE frame.

FIG. 3 shows a PSS and SSS frame and slot structure in the FDD case ofan LTE frame.

FIG. 4 shows a cross sector scenario.

FIG. 5 shows a handover procedure.

FIG. 6 shows an augmented handover procedure according to the presentdisclosure.

FIG. 7 shows cells of a cellular mobile communication system.

DETAILED DESCRIPTION OF THE INVENTION

The present disclosure relates a method and a device for fast handoverin a cross sector scenario in mobile communication systems. The presentdisclosure specifically relates to a method and a mobile device for fasthandover in cellular mobile communication systems.

On implementation relates to a method for fast handover in an E-UTRA/LTEUE mobile device and related mobile device, based on blind measurementof cells that have not been detected by primary cell detection.

One aspect of the present disclosure relates to a method for handover ina mobile communication system, wherein cell search is performed bydetermining cell identities based on cell identity informationtransmitted within downlink data. The cell power of cells detected bycell search is measured. Moreover, the cell power of an additional cell,preferably two additional cells, is measured, the additional cell havinga cell identity that is related to a cell identity of a cell detected bycell search in that the additional cell and the cell detected by cellsearch are adjacent cells.

The method is advantageously applicable in cross-sector scenariosavoiding call drops due to lengthy cell detection.

FIG. 1 shows a base station 11 and mobile device 12 in a mobilecommunication system such as LTE. For base station identification thebase station 11 transmits a cell identity (cell-ID) within the PrimarySynchronization Signal (PSS) and Secondary Synchronization Signal (SSS)of an LTE radio frame 13. The allocation of the PSS and SSS within oneradio frame in time domain is shown for the Frequency Division Duplexcase in FIG. 2 and for the Time Division Duplex case in FIG. 3. Thesequences are transmitted twice within a 10 ms radio frame, hence, every5 ms. There are 504 unique physical layer cell identities in mobilecommunication systems, such as LTE, grouped into 168 groups of threeidentities. Three PSS sequences are used to indicate the cell identitywithin the group and 168 SSS sequences are used to indicate the identityof the group.

Once a cell has been detected, measurements are made in order to make ahandover decision. In a mobile communication system, such as LTE, cellmeasurements are based on cell specific reference signal (CRS) receivepower. CRS are subcarriers multiplexed into a time-frequency grid of adownlink transmission scheme that carrys reference symbols. Referencesymbols are data symbols which are known at the receiver and are usedfor parameter estimation tasks. A plurality of reference symbolsmodulated on CRS form a reference symbol sequence that also carriesunambiguously one of the 504 different cell identities N_(cell-id) aswell as the cyclic prefix (CP) mode as the reference symbol sequence isderived from a pseudo random sequence generated by a scrambling sequencegenerator that is initiated depending on the cell-id.

However, the handover procedure described above causes problems in across sector scenario as shown in FIG. 4. A base station 41 serves threesectors: sector zero 43, sector one 44 and sector two 45. To reduceinterference among the three sectors, the base station normally usesdifferent beam-forming patterns for each sector, potentially with asmall overlapping at the sector boundary. Such a signal isolation methodvia beam-forming serves a very efficient way to mitigate Inter-sectorInterference. However, it also causes handover problems when high-speeduser equipment (UE) 42 moves across the sector boundary 46 as shown inFIG. 4.

The signal from the current sector (sector zero 43) drops rapidly when aUE is leaving for its neighboring sector (sector one 44), while the UEcannot detect and measure the neighboring sector (sector one 44) well inadvance due to signal isolation. Hence, the cell-id of sector one 44needs to be determined and the cell of sector one 44 to be measured in avery short time when the UE is able to receive the beam of sector one44. However, a typical cell detection procedure as described abovetypically takes up some time resulting in a call drop.

In the present disclosure, a method for fast handover in an E-UTRA/LTEUE mobile device and related mobile device, based on blind measurementof cells that have not been detected by primary cell detection isprovided. Generally, a handover procedure involves three blocks inside aUE as shown in FIG. 5. Cell searcher 51 is responsible for finding cellsbased on primary and secondary synchronization sequences inside downlinkdata. Cell list administrator 52 is responsible for cell list managementfor all cells detected by the cell searcher 51 or measured so far. Cellmeasurement engine 53 is responsible for cell measurement based on cellspecific reference signals.

In order to handover to a new sector/cell, the cell searcher 51 needs tofind it in a conventional handover procedure. Once the cell listadministrator 52 gets the new cell's information from cell searcher 51,it will schedule the measurement request with cell measurement engine53. Based on the measurement results, the UE will decide whether to moveservice from current serving cell to one of its neighboring cells.Normally, the cell search will take some time as the synchronizationsignal is periodic

In the present disclosure, handover procedure is augmented by a blindmeasurement based approach in order to speed up handover procedure, thusreducing the call drop rate in the cross-sector scenario. Referring toFIG. 6, an exemplary cellular list is depicted. As shown in FIG. 6,instead of just measuring detected cells 0 to N−1 from the cell searcher51 as shown in FIG. 5, additional cells N and N+1 having the same cellgroup-ID as detected cells are added for measurement.

Also shown in FIG. 6, the cell N−1 is supposed to be the currentlyserving cell and another two cells having same cell group ID as servingcell, denoted as cell N and N+1, are not in the cell list from cell listadministrator 52 in FIG. 5. Referring back to FIG. 3, the sectors,indicated by their sector-id 0, 1 or 2, that are served by the same basestation have the same group-id in a mobile communication system, such asan LTE system. Thus, each sector corresponds to a cell with a specificcell-id being composed of sector-id and group-id.

The measurement engine 53 adds cells having same cell group ID asserving cell, such as cells N and N+1 to the measurement list andperforms measurement blindly for them. With such a blind measurement,the entering sector can be detected and measured in a much faster waysince cell search time is avoided in such a procedure in thecross-sector scenarios, leading to a lower call drop rate. The blindmeasurement can be based on CRS receiver power. Typically, cellmeasurement is much faster than cell detection. Hence, a speed up ofmultiple times is achievable.

Further, additional cells can be added to the cell measurement list bythe cell list administrator 52 instead of by the measurement engine 53.Thus, the measurement engine 53 does not need to be aware of whether thecell in the list is a detected one or a blindly added one.

Instead of just adding two cells sharing the same cell group ID as theserving cell, more cells with same cell group ID as the few strongestneighboring cells can be added for blind measurement. Referring to FIG.7, a group of cells having the same group-id is indicated by solid linedhexagonals 71, while the sectorization is indicated by dashed lines andthe respective sector-id 01, 1 and 2. The base station is Indicated bycapital letters A, B, . . . G and related to a group of cells having thesame group-id. A UE 72 is currently located in cell A0. Thus it isserved by base station A. Assuming the cell searcher detected cells A0,C1, D2, B1 and E1 in the order of their cell power, then the cell listmay be augmented not only with cells A1 and A2 but also with cells C0,C2, D1 and D0 for example, as these additional cells have the samegroup-ID as the cells A0, C1 and D2 of a plurality of cells havingstrongest power.

Simple and effective solution for handover, specifically in cross sectorscenarios, with low computational complexity is therefore provided.

This disclosed method may be applied to any cellular communicationsystem in addition to 4G wireless communication systems. The cellmeasurement is performed blindly for cells in the vicinity of the UE,even though those cells could not have been detected by primary celldetection.

It is therefore an advantage to provide a fast handover procedure toavoid call drops in cross sector scenarios.

What is claimed is:
 1. An apparatus of a user equipment (UE) configuredfor cross-sector handover the apparatus comprising: memory, andprocessing circuitry, configured to: configure a cell searcher to searchfor and detect cells by determining an identity of the cells based oncell identity information conveyed by one or more periodicsynchronization signals to provide a cell list of detected cells, theone or more periodic synchronization signals transmitted periodicallywithin downlink subframes; provide the cell list to a cell listadministrator of the UE; augment the cell list with a cell ID of anadditional cell not detected by the cell searcher, the additional cellbeing associated with a same base station of at least one of the cellson the cell list; configure a cell measurement engine of the UE tomeasure a power level of reference signals of at least some of the cellsof the cell list that have been detected; configure the cell measurementengine to perform a blind measurement of a power level of a referencesignal of the additional cell that had not been detected, the referencesignal being indicative of a cell ID of an associated cell; and initiatea cross-sector handover to the additional cell, that had not beendetected, based on the power level measurement of the associatedreference signal, the cross-sector handover being a handover to a cellserved by the same base station.
 2. The apparatus of claim 1 wherein theone or more periodic synchronization signals include a primarysynchronization signal (PSS) and a secondary synchronization signal(SSS) that are transmitted periodically within the downlink subframes,the one or more periodic synchronization transmitted less often than thereference signals.
 3. The apparatus of claim 1 wherein the referencesignals comprise cell-specific reference signals that are transmittedwithin the downlink subframes.
 4. The apparatus of claim 1 wherein theprocessing circuitry is configured to configure the cell searcher tosearch for and detect cells that have time-aligned frames with acurrently detected cell or a previously-detected cell.
 5. The apparatusof claim 1 wherein the processing circuitry is configured to augment thecell list with cell identities of cells that are served by a basestation serving a cell with a cell identity contained within the celllist, and wherein the power level measurement is based on the cell list.6. The apparatus of claim 1 wherein the identity of the additional cellis further related to the cells of the cell list, wherein the additionalcell and at least one detected cell are served by the same base station.7. The apparatus of claim 6 wherein the identity of the additional cellis further related to an identity of the cells of the cell list whereinat least one detected cell is a currently serving cell.
 8. The apparatusof claim 1 wherein an identity of the additional cell is related to anidentity of the cells of the cell list having maximum cell power.
 9. Theapparatus of claim 1 wherein the processing circuitry is configured toaugment the cell list with the cell ID of the additional cell notdetected by the cell searcher, the additional cell having a same groupID as a group ID of a cell of the cell list having strongest power. 10.The apparatus of claim 1 further wherein the downlink subframes comprisefrequency division duplexed (FDD) radio frames.
 11. An apparatus of auser equipment (UE) configured for faster cross-sector handover, theapparatus comprising: a cell searcher configured to search for anddetect cells by determining an identity of the cells based on cellidentity information conveyed by one or more periodic synchronizationsignals to provide a cell list of detected cells, the one or moreperiodic synchronization signals transmitted periodically withindownlink subframes; a cell list administrator to augment the cell listwith a cell ID of an additional cell not detected by the cell searcher,the additional cell having a same group ID and being associated with asame base station of at least some of the cells on the cell list; and acell measurement engine to measure a power level of reference signals ofat least some of the cells of the cell list that have been detected andto perform a blind measurement of a power level of a reference signal ofthe additional cell that had not been detected, the reference signalincluding a cell ID of an associated cell, wherein the UE is configuredto initiate a cross-sector handover to the additional cell that had notbeen detected based on the power level measurement of the associatedreference signal, the cross-sector handover being a handover to a cellserved by the same base station.
 12. The apparatus of claim 11 whereinthe one or more periodic synchronization signals include a primarysynchronization signal (PSS) and a secondary synchronization signal(SSS) that are transmitted periodically within the downlink subframes,the one or more periodic synchronization transmitted less often than thereference signals.
 13. The apparatus of claim 11 wherein the referencesignals comprise cell-specific reference signals that are transmittedwithin the downlink subframes.
 14. The apparatus of claim 11 wherein theprocessing circuitry is configured to configure the cell searcher tosearch for and detect cells that have time-aligned frames with acurrently detected cell or a previously-detected cell.
 15. A method forcross-sector handover performed by user equipment (UE), the methodcomprising: searching for and detecting cells by determining an identityof the cells based on cell identity information conveyed by one or moreperiodic synchronization signals to provide a cell list of detectedcells, the one or more periodic synchronization signals transmittedperiodically within downlink subframes; augmenting the cell list with acell ID of an additional call not detected by the searching, theadditional cell being associated with a same base station of at leastone of the cells on the cell list; measuring a power level of referencesignals of at least some of the cells of the cell list that have beendetected; performing a blind measurement of a power level of a referencesignal of the additional cell that had not been detected, the referencesignal indicative of a cell ID of an associated cell; and initiating across-sector handover to the additional cell that had not been detectedbased on the power level measurement of the associated reference signal,the cross-sector handover being a handover to a cell served by the samebase station.
 16. The method of claim 15 wherein the one or moreperiodic synchronization signals include a primary synchronizationsignal (PSS) and a secondary synchronization signal (SSS) that aretransmitted periodically within the downlink subframes, the one or moreperiodic synchronization transmitted less often than the referencesignals.
 17. The method of claim 15 wherein the reference signalscomprise cell-specific reference signals that are transmitted within thedownlink subframes.
 18. The method of claim 15 wherein the searchingcomprises searching for and detect cells that have time-aligned frameswith a current serving cell or a previously-detected cell.